The present invention relates to a process for the preparation of a dispersion of crystalline nano-particles, particularly a dispersion of crystalline nano-particles in an aqueous medium and more particularly to a process for the preparation of a dispersion of crystalline nano-particles comprising a substantially water-insoluble pharmacologically active compound in an aqueous medium.
Dispersions of a solid material in a liquid medium are useful in a number of applications including paints, inks, dispersions of pesticides and other agrochemicals, dispersions of biocides and dispersions of pharmacologically active compounds.
In the pharmaceutical field many pharmacologically active compounds have very low aqueous solubility which can result in low bioavailability when such compounds are administered to a patient. Generally, the bioavailability of such compounds is improved by reducing the particle size of the compound, particularly to a sub-micron size, because this improves dissolution rate and hence absorption of the compound.
The formulation of a pharmacologically active compound as an aqueous suspension, particularly a suspension with a sub-micron particle size, enables the compound to be administered intravenously thereby providing an alternative route of administration which may increase bioavailability compared to oral administration.
Formation of suspensions of crystalline nano-particles (nano-crystals) through direct precipitation is known in the art to be problematic. The rapid precipitation necessary to achieve small particle size often results initially in a suspension of amorphous material. Although the amorphous particles will often crystallise over time, slow crystal growth tends to result in the formation of large crystals with a particle size of>1 micron. Attempts to precipitate crystalline material by direct precipitation is generally difficult to control and results in the formation and growth of large (>1 micron) crystals.
U.S. Pat. No. 4,826,689 describes a process for the preparation of amorphous particles of a solid by infusing an aqueous precipitating liquid into a solution of the solid in an organic liquid under controlled conditions of temperature and infusion rate, thereby controlling the particle size. U.S. Pat. No. 4,997,454 describes a similar process for the preparation of amorphous particles in which the precipitating liquid is non-aqueous. U.S. Pat. No. 5,118,528 also describes a process for preparing a colloidal dispersion of particles using a solvent/anti-solvent precipitation process.
U.S. Pat. No. 5,780,062 describes a process for preparing small stable particles wherein a solution of a substance in an organic solvent is precipitated into an aqueous solution containing polymer/amphiphile complexes.
Generally the amorphous solubility of a substance is significantly higher than the crystalline solubility of the substance. Accordingly, amorphous particles are prone to higher rates of particle growth through solubility driven particle growth mechanisms such as Ostwald ripening compared to crystalline particles. Therefore, crystalline suspensions tend to be stable for significantly longer than dispersions of amorphous particles because the Ostwald ripening occurs at a slower rate in the crystalline particles. Amorphous particle dispersions are also prone to re-crystallise as a more stable crystalline form resulting in the uncontrolled growth of large crystals.
WO98/23350 and WO99/59709 describe processes in which a melt of an organic compound is dispersed in a liquid to form an emulsion. The emulsion is then subjected to ultrasound to give a crystalline dispersion. The particles prepared using the process are of the order 2 to 10 microns.
Crystalline dispersions obtained directly by precipitation are known in the art to be influenced by agitation of the solutions. Various methods of agitation are known in the art (see for example, WO 01/92293), for example mechanical mixing, vibration, micro-wave treatment and sonication.
WO 96/32095 describes a process for the direct formation of crystals by introducing a solution of a substance in a solvent in a droplet form or as a jet into an agitated anti-solvent. Agitation is achieved using a number of techniques including ultrasonic agitation. The resulting crystals generally have a mass median diameter of 1 to 6 microns.
U.S. Pat. No. 5,314,506 describes a crystallisation process in which a jet of a solution containing a substance is impinged with a second jet containing an anti-solvent for the substance. The rapid mixing produced by the impinging jets results in a reduction of the crystals so formed compared to conventional slow crystallisation processes. The smallest crystals disclosed are about 3 microns and the majority are in the range of from 3 to 20 microns.
WO 00/44468 describes a modification to the apparatus described in U.S. Pat. No. 5,314,506 wherein ultrasound energy is applied at the point of impingement of the two jets to further enhance localised mixing and is stated to give direct formation of small crystals with a diameter of less than 1 micron. Generally the crystalline particles described have an average size of 0.5 microns.
WO00/38811 describes an apparatus and process wherein crystalline particles suitable for inhalation are prepared by precipitation of a substance from solution using an anti-solvent in a flow-cell mixing chamber in the presence of ultrasonic radiation at the point of mixing the solvent and anti-solvent system. This method results in the direct crystallisation of particles typically having an average particle size of from 4 to 10 microns. WO02/00199 and WO03/035035 describe modifications to the process described in WO00/38811 which reduce crystal agglomeration and enable more efficient isolation of the crystals so formed.
In a novel method of obtaining crystalline nano-particles, Kasai et al (Jpn. J. Appl. Phys., 31, L1132 (1992)) precipitated particles by dropwise addition of an ethanol solution of an organic compound (typically 50 μl with a concentration of 30 mM) into 10 ml of vigorously stirred water giving a total concentration of approximately 0.15 mM. Stirring was then continued for a few minutes and the particle size obtained was about 300 nm. They found that the particle size could be reduced by precipitation at still lower concentrations. By the same procedure Kasai et al (Bull Chem Soc Jpn, 71, 2597 (1998)) formed aqueous suspensions of nano-crystals of perylene at concentrations between 2.5 and 20 μM. However, such low concentrations generally require the sample to be concentrated e.g. by ultrafiltration, before use. Furthermore, if the total initial concentration of organic compounds is increased then the size of particles obtained by such methods is>1 μm. (see e.g. F. Ruch, E. Matijevic, Journal of Colloid and Interface Science, 229, 207 (2000)).
EP 275 607 describes a process wherein ultrasound energy is applied to a suspension of crystals in a liquid phase, the ultrasound being used to fragment the pre-formed crystals. Generally the volume mean diameter of the resulting crystals was 10 to 40 microns.
An alternative approach to direct precipitation is to reduce the particle size of the material prior to suspension, for example by milling as described in U.S. Pat. No. 5,145,684, however this can be disadvantageous as it may be difficult to achieve a sufficiently uniform crystal size. It is particularly important that the particle size in a dispersion of a pharmacologically active compound is as uniform as possible because a difference in particle size is likely to affect the bioavailability and hence the efficacy of the compound. Furthermore, if the dispersion is required for intravenous administration, large particles in the dispersion may render the dispersion unsuitable for this purpose, possibly leading to adverse or dangerous side effects.
There is therefore a need for alternative processes that enable nano-crystals to be formed, particularly nano-crystals of less than 500 nm, more particularly less than 400 nm, especially less than 280 nm and more especially less than 250 nm with a narrow particle size distribution.